15 April 2011

Space Frame Joining Techniques - Adhesive Bonding

Adhesive bonding tends to be considered as a low cost option for joining spaceframes, but process automation requirements and the need for viscosity compensation to provide consistent application tend to contradict this view.

There are however a number of advantages associated with the use of adhesives for the joining of structural components that leads to the use of this method being retained particularly with respect to joining aluminium spaceframes.

  • No distortion as would typically result from arc welding
  • Improved joint stiffness due to the continuous bond, as opposed to local joint contact, and more uniform stress distribution.
  • Good energy absorbing characteristics combined with noise and vibration dampening properties.
  • Dual purpose, provides mechanical strength and seals against moisture and debris ingress.
  • Smooth joints reduce stress concentration at the joint edges providing good fatigue resistance.
  • High strength in shear.
  • Dissimilar metals can be joined without leading to galvanic corrosion.
The limitations associated with adhesive bonding are numerous but the main factors that effectively rule it out as a stand alone process relate to the inability to use non destructive methods to check the strength of the bond and the unknown effects of exposure to different environments over time.

Clearly it would be very risky to rely entirely on the adhesive bond without knowing its real strength or understanding how it will endure in all potential environments it might be exposed to. There are also further limitations as listed below: -

  • Epoxy or solvent based adhesives typical of the type used can be hazardous to health and require suitable fume extraction systems, protective clothing and storage facilities to protect against the risk of fire.
  • Investment in equipment is risky because there is a possibility that these substances may be banned in the future.
  • Heat curing is a necessary stage in the process.
  • Limited shelf life of adhesives requires that adequate batch management procedures are in place.
  • Adhesive dispensers require regular routine maintenance to keep them clean.
  • For aluminium, surfaces need to be carefully prepared to ensure a good bond is achieved.
Despite all the limitations adhesive bonding is still being adopted, however automobile manufacturers have typically elected to provide some sort of mechanical re-enforcement that can combine with the use of adhesives and provide solutions to the inherent problems associated with this method of joining.

Audi for example, among other methods, use a joining technique called roller type hemming where rollers secured to a robot arm bend an outer panel over an inner panel which when combined with a hem-bonding adhesive provides a powerful connection. These adhesive joints are then hardened using a process called ‘inductive gelling’ which uses an electric field to target the specific hem-bonded zones.

Ref. Youson M (2002)

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